This application relates to measurements of polarization of light.
The state of polarization of light is an important parameter of an optical beam in part because it affects behavior of the optical beam when interacting with an optical medium or an optical element. Various optical devices and systems can be sensitive to the state of polarization of the beam to be processed or transmitted. For example, certain coherent optical processing may require a match between the states of polarization of two separate optical beams when the two beams are superposed. For another example, a birefringent optical element may attenuate an optical signal differently when the polarization of the signal forms different angles with respect to a given principal axis of polarization of the element. An optical amplifier with a saturable gain medium may also produce a polarization-dependent gain when a polarization component with a high intensity saturates the gain medium and hence experiences an optical gain less than that of another, weaker polarization component. Furthermore, certain optical modulators may also produce different modulation depths on optical signals with different polarizations. Semiconductor electro-absorption modulators and electro-optical modulators based on birefringent crystals such as lithium niobate are examples of such modulators.
Hence, it is desirable to control the polarization of an optical signal in those and other polarization-sensitive devices and systems. To achieve such polarization control, it is essential to measure the state of polarization of the signal so that a proper polarization control can be applied in response to the measured polarization. Various polarimeters have developed to measure the state of polarization of light based on analysis of the Stokes polarization vector. Such polarimeters may be designed to split light into four different beams for measuring the Stokes vector components.
In one implementation, for example, a first beam is used to measure the total intensity of the light; second and third beams are sent through polarizers at different relative angles where the transmitted intensities are measured; and a fourth beam is sent through a phase retarder and a polarizer where the transmitted intensity is measured. The measured intensities of the four beams are then used to compute the four Stokes vector components which uniquely determine the state of polarization.
The polarization of an optical signal may not be static but dynamically vary with time in some optical systems due to fluctuations in factors such as light sources, optical components, and optical transmission media. For example, some optical fibers may be birefringent to exhibit different refractive indices for different polarizations. Typical causes for this fiber birefringence include, among others, imperfect circular cores, and unbalanced stress in a fiber along different transverse directions. Fluctuations in local temperature and stress along a fiber line, therefore, can randomly change the axis of birefringence of the optical fiber at different locations. The polarization of light transmitting through such a fiber, therefore, may also fluctuate with time. This can also cause polarization-mode dispersion (PMD) in optical signals with two orthogonal principal polarization states.
Hence, it may also be desirable that the polarimeter operates sufficiently fast so that a polarization control mechanism can change its control in response to any variation in the input polarization of light and therefore maintain the output polarization at a desired state.